Aqueous based hydraulic fluids are well known in the art as exemplified, for example, by U.S. Pat. Nos. 3,580,847, 4,151,099, 3,657,123, 3,005,776, 2,944,976, and 4,257,902. These aqueous based hydraulic fluids are used in various areas as mechanical power or pressure transmitting media in situations where lubricating and wear preventing characteristics are necessary. In the technology of hydraulic power transmission, mechanical power is imparted to a fluid called a hydraulic fluid in the form of pressure by means of a hydraulic pump. Power is utilized where desired by tapping a source of the hydraulic fluid and transforming the power as pressure back to mechanical motion by a hydraulic motor. The hydraulic fluid is utilized as a pressure and volume transmitting medium and any non-compressible fluid can perform the function. Water is the oldest fluid used for this purpose and is still sometimes used without additives. In the prior art, however, there has been heavy emphasis on the development of aqueous based fluids which contain various types of additives and which are designed and manufactured specifically for specialty used.
Within any hydraulic system there are found such metals as iron, brass, solder, bronze, aluminum, zinc, cadmium and other commonly used metals and the presence of several metals in a hydraulic system provides the possibility for the formation of various corrosion problems. Therefore, such fluids traditionally contain corrosion inhibitors.
Such fluids also generally contain emulsifying agent or dispersants and various other additives such as thickening agents, freezing point depressants, antiwear agents, and various other additive designed to meet problems encountered in use of the hydraulic or functional fluids.
It is also known that lubricating compositions including power transmitting fluids can contain hydrocarbon substituted succinic acid esters wherein the hydrocarbon substituent is an aliphatic carbon chain containing a large number of carbon atoms. Such products are disclosed, for example, in U.S. Pat. No. 3,381,022 which describes oil soluble esters, diesters and mixtures of substantially saturated polymerized olefin substituted succinic acid with mono or polyhydric aliphatic alcohols having up to 40 carbon atoms wherein the polymerized olefin substituent has at least about 50 aliphatic carbon atoms and a molecular weight of about 700 to 5000, with no more than 5% olefinic linkages. These succinic acid esters are described as being useful as additives in lubricants to impart detergent properties to the lubricant particularly under high and low temperature conditions. There is no disclosure, however, for incorporation of these lubricants into water based hydraulic fluids.
U.S. Pat. No. 4,368,133 describes aqueous systems containing functional additives which contain solubilizers made by reacting an alkyl substituted succinic anhydride containing about 12 to 500 carbon atoms with an alkanolamine. These compositions are water based hydraulic fluids and the succinic anhydride-alkanolamide reaction products are present as solubilizing agents.
U.S. Pat. No. 4,440,902 describes diesters of alkenyl succinic acids and ethylene oxide/propylene oxide block polymers and their use as demulsifiers for oil water emulsions and as corrosion protective agents and dispersants for dyestuffs. Similarly, U.S. Pat. No. 3,957,854 describes ester-containing compositions useful as additives in normally liquid fuels and lubricants wherein the oil soluble carboxylic acid esters are prepared by reacting an aliphatic hydrocarbon polycarboxylic acid acylating agent containing at least 30 aliphatic carbon atoms with at least one polyalkylene glycol which is a demulsifier for an aqueous emulsion, and at least one primary or secondary amine.
U.S. Pat. No. 4,036,772 discloses esters formed from high molecular weight carboxylic acids derived from low molecular weight ethylenically unsaturated carboxylic acid reagents and oxidized ethylene propylene interpolymers with mono or polyhydric alcohols. These esters are said to be useful in lubricant compositions and normally liquid fuel compositions.
It is also known from U.S. Pat. No. 4,256,605 and related U.S. Pats. Nos. 4,504,275, 4,504,276, and 4,509,950 that compositions useful as surfactants in the emulsification of oils in aqueous media may be obtained by blending two types of polyesters which are the products of condensation of an alkenyl succinic anhydride with a polyalkylene glycol. One type of polyester is derived from an anhydride in which the alkenyl group contains 40 to 500 carbon atoms, such as polyisobutenyl succinic anhydride, and the other type is derived from an anhydride in which the alkenyl group contains 8 to 18 carbon atoms. These compositions are said to be superior to the use of the polyester separately in providing emulsions of high stability produced with a low energy output. As acknowledged in column 1 of these patents, British Patent No. 1,055,337 discloses esters obtained by the reaction of saturated hydrocarbon substituted succinic anhydrides containing at least 50 aliphatic carbon atoms in the substituent group with a polyhydric alcohol such as an alkylene glycol or polyalkylene glycol, and the use of such esters as additives for the purpose of imparting detergent properties to lubricant. This British patent corresponds to U.S. Pat. No. 3,381,022, discussed herein.
U.S. Pats. Nos. 3,045,042, 3,255,108, 4,481,125, 4,447,348, 3,419,665, 3,640,872, 2,993,773, 3,579,453, and 3,694,176 also disclose succinic acid esters wherein the ester moiety is a hydrocarbon or substituted hydrocarbon, and use of such esters in aqueous based hydraulic fluids of various types.
In none of this known prior art, however, are there to Applicant's knowledge description of substituted succinic acid esters and resulting aqueous based hydraulic fluids which are particularly useful in having high stability in hard water and in other areas where high water content hydraulic fluids are used.